Recently, wind turbines have received increased attention as an environmentally safe and relatively inexpensive alternative energy source. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Generally, a wind turbine includes a plurality of blades coupled to a rotor through a hub. The rotor is mounted within a housing or nacelle, which is positioned on top of a tubular tower or base. Utility grade wind turbines (i.e. wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., thirty or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives the rotor of one or more generators, rotationally coupled to the rotor.
During operation of at least some known wind turbines, rotor speed is controlled to prevent the wind turbine rotor from rotating at or above a predetermined speed that may damage components of the wind turbine. For example, at least some known wind turbines control rotor speed and/or power by pitching the rotor blades using a pitch drive system that changes an angle of the rotor blades, thereby changing the aerodynamic torque of the rotor. Since their introduction, wind turbines have continuously increased in physical size and electrical power output. However, as rotor diameter and therefore rotor blade lengths increase, loading on the blades and rotor system increases, and friction within the pitch drive system may also increase, both of which may increase the torque required by the pitch drive motor to pitch the rotor blades.
Wind turbines can experience operational conditions where the capability of the pitch system is such that the blade(s) cannot follow the commanded pitch angle. Variable pitch control is required in order to limit maximum power output and alleviate wind loads on the turbine. Due to variation in the pitch system components and operating environment factors including electrical system limitations, pitch bearing mechanical limitations, blade mechanical limitations, and wind conditions and loads, faults due to blade positions too far from command points can occur, which can lead to turbine unavailability during relatively high wind (high power) conditions.
Prior methods of addressing the difficulty in moving wind turbine blades during blade angle operations include increasing the continuous or maximum power and current capability of the pitch system, increasing gearbox ratio between the pitch motor and blade bearing, replacing hardware such as bearings, and modification of turbine controls.
However, a cost-effective way is needed to increase the peak torque capability for existing and new turbines that may have problems under certain operating conditions in moving blades per the control command. In practice, this peak torque capability may only be needed a small fraction of operating time.
Thus, an objective of the present disclosure is to provide a cost effective method to increase peak torque capability for existing wind turbines that have problems, under certain operation conditions, moving blades according to a control command.